1. Field of the Invention
The invention relates to an acrylate-based optical coating composition comprising an organic peroxide initiator which presents a low yellowness level. More particularly, the composition is applied to a lens and cured in the mold to form a coating with reduced yellowness having high adhesion, abrasion-resistant, or photochromic properties.
2. The Prior Art
Injection molded polycarbonate ophthalmic lenses are lightweight and possess excellent mechanical properties. Photochromic dyes are incorporated into certain lenses to enhance their optical properties by automatically lightening or darkening based on the amount and type of light that they receive. Since the photochromic dye molecules can decompose when exposed to the heat of the injection molding process, such dyes need to be introduced on to the lens after the injection molding step. One method is to incorporate the photochromic dye into a coating composition and apply it to a lens surface via dip coating or spin coating. The surface discontinuity of bifocal and trifocal segmented lenses are not well suited to these typical coating processes. The viscous coating material builds-up at the segment which results in an undesirable darker color at the segment when exposed to UV light. A prior art process utilizing a photochromic polyurethane coating is shown in U.S. Pat. Nos. 6,187,444 and 7,258,437. These approaches do not use organic peroxides in the coating compositions, rather they utilize condensation reactions.
Photochromic (PhCh) Semi-finished Straight-Top (SFST) lenses can be made by injection molding polycarbonate (PC) behind a photochromic wafer. The wafer can be a tri-layer of PC/PhCh/PC. VisionEase U.S. Pat. No. 7,036,932 and U.S. Published Patent Application 2007/0122626 describes such a product.
Another method is to over-mold a photochromic coating onto a clear substrate or a clear coating onto a photochromic substrate. In one example, U.S. Pat. No. 6,863,844 describes a composite lens utilizing an acrylic photochromic monomeric solution and curing it by UV. These UV cure processes describe using UV radiation wavelength outside the absorption of the photochromic or employ filters to minimize UV absorption. This helps to protect the photochromic and ensure a good cure. In another example, U.S. Pat. No. 7,258,437 describes over-molding a non-photochromic polyurethane-acrylic solution on top of a photochromic polyurethane coated substrate by UV.
Some general photochromic acrylic coatings are cured as described in U.S. Pat. No. 6,602,603 utilizing UV initiators and azo initiators. The photochromic coatings utilizing the azo thermal initiator was cured in a vacuum oven, flushed with nitrogen and cured by IR for 4 minutes reaching a maximum temperature of 124° C. U.S. Pat. No. 5,621,017 describes the UV cure of an acrylic photochromic composition.
Other such photochromic coatings are Aminoplast melamine PhCh coatings, mentioned in U.S. Pat. Nos. 6,506,488 and 6,432,544. Thermal initiators are used to prepare polymers, while acid catalysts (eg. p-TSA) cure the photochromic Cymel resin mixture. A photochromic epoxy coating is described in U.S. Pat. No. 6,268,055. Thermal initiators are only used to prepare the polymers, no peroxide initiator is used to cure the epoxy PhCh coating.
Yet another method is proposed in U.S. Published Patent Application 2007/0138665 where a thermoplastic segmented lens substrate is injection molded. The mold block opens and a coating liquid is applied to the front surface of the segmented thermoplastic lens. The mold block then closes. This serves two (2) purposes. First, when the mold block closes, it spreads the coating over the front (CX) surface of the segmented thermoplastic lens to provide a uniform thickness. Secondly, the mold block will provide suitable heat to cure the coating formulation. A thermal initiator (t-butylperbenzoate) and a catalyst (cobalt naphthenate) are utilized to ensure a good cure. When the coating is a photochromic coating, the organic peroxide thermal initiator (t-butylperoxybenzoate) appears to damages the photochromic molecules resulting in a visible yellow color. Accordingly, there is a need to produce coatings with low yellow color. More particularly, it would be desirable to provide a photochromic coating composition with reduced yellowness, and a method for in-mold coating, especially for coating bifocal or trifocal segmented lenses.